Mix all the meatball ingredients together with plenty of seasoning, then shape into walnut-sized balls. Freeze for 30 mins to firm up if the party is about to start, or chill in the fridge for up to 24 hrs.
Heat oven to 220C/200C fan/gas 7. Roll the meatballs in the oil in a baking tray, then roast for 20 mins until browned and cooked through.
When the meatballs are nearly ready, mix the vinegar and sugar in a mixing bowl to dissolve, then stir in the cucumber and dill, if using.
Split the gluten free rolls or subs and spread with a little cranberry sauce. Add some cucumber and 2-3 meatballs, arrange on a platter and serve.